The art of hydraulic elevators is undergoing a period of change. Conventional in-ground buried hydraulic cylinders used to motivate elevators in relatively low buildings (e.g., 2-6 floors) are being phased out in many areas due to environmental concerns over fluid leaking from the cylinders. Several replacement systems have been adopted. These systems include double post hoist-away hydraulic cylinders, telescopic cylinders, and roped (cable hydraulic) cylinders wherein pistons are placed beside the elevator car and unfortunately take up valuable hoist-away space. Previously, cylinders were placed below the elevator car and in-ground directly beneath the elevator car so as to minimize space requirements.
The use of these types of new cylinders has created new design parameters for the hydraulic seals used in the cylinders. Cross-sections of these seals need to be narrower to accommodate tighter clearances between the cab and piston. Indeed, dual piston hydraulic elevators place more surface area of the rubber seals in contact with the piston than single piston hydraulic elevators. The area of contact is approximately 1.4 times greater for dual pistons as compared to single pistons, weight and pounds per square inch being equal.
Cable hydraulic elevators operate at a 2-1 to 4-1 ratio, which means the piston must operate smoothly at one-half to one-quarter the speed of the car. Most real pistons in a hydraulic elevator will operate smoothly to below one foot per minute for short periods of time without vibration (one to two seconds) as piston speed decreases in cable hydraulic elevators. Seals must operate for longer periods of time below one foot per minute without causing vibration ("jumpiness" or "jerkiness") to the elevator cab and alarming passengers inside the cab.
Two (2), three (3), and four (4) stage synchronous hydraulic pistons operate under similar if not identical conditions described in the operation of cable hydraulic elevators.
By their nature, elevator hydraulics require very long hydraulic pistons. Almost all elevator hydraulics are made from pipe that are centerless ground. Tolerances on large sections of pipe fastened together, then ground and polished to a finished dimension, vary more than similar but smaller hydraulics. It would therefore be advantageous for the same seal to have the ability to seal over a wider range of tolerances with lower friction than previously possible with conventional seals.
For a standard seal cavity having a vertical cross-sectional width of 0.500 inches, most conventional lip seals operate with a cross-sectional interference of about 0.080 inches. From data gathered from controlled experiments, it has been observed that increasing interference increases the break-away force needed to overcome friction between the seal and the hydraulic piston. It has also been observed that decreasing interference lowers break-away, but also decreases seal life and sealing efficiency. These are the opposing design considerations of a hydraulic elevator seal.
Many in the art of hydraulic elevator seals have endeavored to overcome the problem of rapid failure or rupture of the seal. Seal devices which include un-reinforced sealing lips are more likely to fail due to splitting or tearing than fabric reinforced seals and the like. Such seal devices can include a reinforced base that fits within a seal cavity when the seal device is installed. But, the movable member in a hydraulic elevator system contacts such seal devices at the un-reinforced sealing lips only. Thus, such seal devices have low friction characteristics but are undesirable due to the splitting or tearing phenomenon. An example of such a seal is described in U.S. Pat. No. 5,509,670 issued to Wheeler on Apr. 23, 1996, and assigned to The Texacone Company.
Seals which include sealing lips made completely of a rigid re-enforcing fabric are known to resist sudden failure. That is, these seals fail in a gradual fashion, allowing time for inspection and replacement. However, such seals generally have higher break-away requirements and are thus prone to "jerkiness" and "jumpiness" when placed in operation, and such characteristics are undesirable.
What is needed, then, is a low friction, fluid seal device that resists rapid or sudden failure or rupture. Such a fluid seal device is lacking in the prior art.